Calculate your handlebar position relative to frame reach and stack

August 7, 2026

by Paul Vousden

Decoding Your Bike Fit: How to Calculate the Perfect Stem and Spacers

You know the dilemma. You want to buy a new bike, and you can easily look up the frame's stack and reach online. But how do you figure out exactly what stem length and how many spacers you need to match your current, perfectly dialed-in riding position?

As we covered in our latest video, measuring your current bike's true X and Y coordinates (Reach and Stack to the center of the handlebars) is step one. All you need is a wall, a tape measure, and a calculator. But once you have those golden numbers, things get complicated.

To bridge the gap between a bare frame and your ideal handlebar position, we have to do some reverse-engineering. Grab your calculator—here is exactly how to run the math.

The Variables You Need

Before you start crunching numbers, gather these figures:

Target Reach (T $X$ ) & Target Stack (T $Y$ ): The ideal handlebar coordinates you measured from your current bike.
Frame Reach & Frame Stack: The geometry numbers of the new frame you want to buy.
HTA: The Head Tube Angle of the new bike (e.g., 72.5°).
Stem Angle: The angle of the stem you plan to use (usually -6°).
Hidden Hardware: The height of the headset dust cover plus half the height of the stem clamp. For modern integrated cockpits, this is usually between 20mm and 26mm.

Step 1: Find the Gaps and the True Angle

First, calculate the horizontal and vertical distance you need to bridge between the new bare frame and your target handlebar position.

Delta X = Target Reach - Frame Reach

Delta Y = Target Stack} - Frame Stack

Next, find your stem's true upward tilt relative to the ground. Because the steerer tube leans backward, a negative stem actually points slightly up!

ANGLE effective = (90º - HTA) + Stem Angle

Note, stem angles are usually negative, but not always.

Step 2: Calculate the Stem Length

Now we use trigonometry to find exactly how long your stem needs to be to close that gap, accounting for the angles of both the steerer tube and the stem.

Note: The calculator will give you a very specific decimal. Since stems are only sold in 10mm increments, round to the nearest available size (e.g., 100mm or 110mm).

When using sin and cos on your calculator be sure to be in deg mode.

Step 3: Calculate the Spacers

Finally, we calculate how much hardware needs to sit on the steerer tube to push that stem up and back into your perfect position. First, find the total steerer height:

Total height of all hardware and spacers required is this equation

Remember that "Hidden Hardware" trap from the video? Your stem doesn't clamp directly to the bottom of that measurement. To find out how many physical spacer rings you need to buy, subtract your headset cover and half-clamp height from the total:

Spacers Needed = Total Steerer Height - Hidden Hardware

Wrap Up

And there you have it! By running these equations, you can confidently order the correct integrated handlebar or stem/spacer combo before your new bike even arrives.

If this math makes your head spin, don't worry. Your bike fitter will love you if you just bring in your base measurements! If you are commissioning a custom build with us here at Mapdec, we do all this heavy lifting for you to ensure your new bike fits like a glove from day one. Get in touch via the links below, and we look forward to hearing from you!